In Outlook 2010 you can turn on the quota status bar by right clicking the Outlook status bar at the bottom and selecting Quota Information. This will add the graph in the bottom left corner letting the user know how much of their quota they are consuming.

You also click on File and select Info. Next to mailbox cleanup it will display how much of your quota you are using.

In OWA you will be able to see the size of your mailbox and quota by hovering your cursor over your mailbox name.